Yaakov marrying two sisters according to the tosafists

In order to answer the questions of how Yaakov could marry two sisters, or his children marrying their twins (according to the midrash), Tosafot (found in Tosafot HaShalem Genesis 37:35) suggest that non-Jews (B'nei Noach) are only prohibited in the erva of adultery! And they base it off a statement of Rabbi Chanina in Snahedrin 57b that gentiles only have the prohibition of "בעולת בעל", implying anything else they do not have. Am I reading this wrong? True, they have difficulties with that answer afterward, but it is a suggestion.